Surgery, particularly minimally invasive surgery, often requires the use of retractors. A retractor is a medical instrument that, when inserted in an incision or opening in the body, is used to separate tissue to permit better visualization and access to the underlying target of the surgery. Retractors will oftentimes expand an initial skin incision to more than twice the original size and cause compression to the surrounding tissue (neural, vascular, and other soft tissue structures) in the process. Minimally invasive surgery offers the advantages of decreased trauma, less post-operative pain, faster recovery, and a smaller surgical scar. Yet studies have shown that the learning curve for minimally invasive surgical techniques can be steep for even an experienced spine surgeon. This leaves the possibility of complications and unnecessarily long surgeries during the learning phase prior to the development of an efficient technique. A minimally invasive procedure will normally require a specialized retractor. These specialized retractors allow good visualization of the target tissue but, due to the small incision, do not allow adequate visualization of the surrounding structures. It is known that excessive compression or traction on neural structures as a result of using a retractor may result in damage and permanent loss of nerve function, which translates into pain, numbness, and weakness post-operatively.
In lateral access spine surgery, an incision is made in the flank and dissection is performed through the retroperitoneal space and then through the psoas muscle to allow access to the spinal column. A retractor is advanced through this same tract and then expanded in the psoas muscle, revealing the spinal column. The psoas muscle is traversed by the lumbar plexus (the neural bundle that supplies the leg), and anatomic and clinical studies have shown that branches of the lumbar plexus are in potential jeopardy during a transpsoas spinal approach. Patients undergoing lateral access transpsoas spine surgery have been reported to have neural deficits post-operatively thirty percent of the time. It is therefore necessary to identify contact and subsequent compression or damage of a nerve(s) as it occurs during placement and expansion of the retractor and throughout the course of a lateral access surgery in the spine. Current intraoperative neuromonitoring systems allow for testing of neural structures using a medical instrument that is used in performance of the surgery. U.S. Patent Application Publication No. 2005/0182454 discloses, for example, a surgical system for electrically stimulating nerves via one or more stimulation electrodes at the distal end of surgical access components while monitoring the EMG responses of the muscle groups innervated by the nerves. This is accomplished via 8 pairs of EMG electrodes placed on the skin over the major muscle groups on the legs. U.S. Pat. No. 6,466,817 discloses a system for detecting proximity to a nerve and status of a nerve by providing a stimulus to a calibration electrode and a proximity electrode. The system includes EMG electrodes to receive the response after a stimulus is applied to the calibration or proximity electrode. The calibration electrode is positioned near a nerve. The proximity electrode may be coupled to any medical device including a cannula, needle, catheter, ablation device, laser, etc.
The above types of tests often give non-specific neurodiagnostic changes during a surgery, which can be open to interpretation. A neural pathway will conduct electrical impulses from one end to the other whether created internally by the patient or created externally by a method of neuromonitoring. The signals can further degrade over increased lengths of a given pathway and as the signals cross a synapse (nerve-nerve interface).
In contrast to prior art methods, the disclosed method and apparatus include testing shorter segments of a neural pathway and, in contrast to providing stimulation from a medical device/electrode at the surgical site, the disclosed method tests specifically, across a site of suspected injury or surgical site, such as proximal to distal and vice versa. A shorter neural segment will give a more reproducible reliable response.